1. Field of the Invention
This invention relates in general to methods and means for removing charged and non-charged particles from fluid. More particularly, it relates to methods and means for removing charged and non-charged particles from fluid by means of a filter containing a pyroelectric filtering medium.
2. Description of the Prior Art
Particle filters have been used in the past to remove particulate matter from various fluid media. For example, they have been used as filters in fans, dust suction equipment, motor exhausts, cigarettes, gas masks, air filters, furnace filters, etc. Generally, particle filters may be categorized as either mechanical filters or electrically charged (electrostatic) filters.
Mechanical particle filters are designed to remove particulate matter by means of special mechanical structures and combinations of filtering media. However, such filters are often of limited usefulness because if fine filtering action is necessary, they become clogged with particulate matter relatively quickly and thereafter offer a great deal of resistance to fluid flow.
Electrostatic filters are designed to remove particulate matter by means of electrical attraction between the particles to be removed and the filtering medium. They may be made entirely from an electrostatically charged filtering medium, or optionally they may include a filtering medium combined with various other components, such as fibers or filaments to form a woven filter. Additionally, they must be used in various configurations such as webs, parallel layers of films, spheres, etc.
Known electrostatic filter media are generally formed of dielectric materials often referred to as electrets and made from such substances as carnauba wax, insect wax, spermaceti, acrylics, polystyrene, and various ferroelectric materials. When electrets are polarized, a static electric charge is produced on the surfaces thereof. The five most common types of polarization are atomic polarization, dipole polarization, interfacial polarization, space-charged polarization, and external polarization. These types of polarization are semi-permanent and are more fully discussed in Electret Devices for Air Pollution Control (T. Kallard ed. 1972).
Atomic polarization occurs when the negative electronic cloud of a dielectric material is displaced relative to its positive nucleus as a result of the application of an electric field that produces a small electric moment having a short lifetime. Dipole polarization occurs when the molecules of a dielectric material that have an electric moment align along the direction of an applied electric field producing an electric moment of the entire dielectric. Interfacial polarization occurs when an electric field is applied to a non-homogeneous dielectric material. Free charge carriers in the non-homogeneous dielectrics are relatively free to move only within single microscopic domains. Consequently, the carriers build up along the barriers of the domains resulting in the creation of a dipole moment. Space-charged polarization occurs from ionic conduction currents in homogeneous dielectric materials and the formation of space-charge clouds in the electrode regions of an applied electric field. External polarization occurs from the injection of equal and opposite charges on opposing surfaces of a dielectric material.
Polarization may be most effectively accomplished by heating the dielectric material to increase its conductivity, applying a high voltage electric field to it, cooling it to room temperature while under the influence of the electric field, and removing the electric field. Application of the electric field causes the displacement of the electron cloud, alignment of the dipoles, movement of the free charge carriers, or the injection of the electrons into the dielectric. The higher the temperature of the dielectric the easier it is for polarization to occur. When the dielectric is cooled in the presence of the electric field, the electrically induced polarization is semi-permanently held therein.
Although prior art electrostatic filters have served satisfactorily in operation, they suffer from an inherent deficiency in having only limited lifetimes. Some of these filters have electret materials capable of holding their electrostatic charge for periods of time of up to about two years, but the electric field of such electret filters decreases in intensity with time and results in decreased filtering capabilities. More importantly, the electric field is also decreased as particles are attracted to the filter. Moreover, care must be taken in the preparation and handling of prior art filters to prevent neutralization of oppositely charged surfaces of their electrets or adjacent members, for example, by the creation of a conductive path.
Various electrostatic filters of the type described above are disclosed in U.S. Pat. Nos. 2,724,457; 2,740,184; 3,193,912; 3,307,332; 3,463,168; and 3,487,610. However, such patents do not disclose the use of a molecularly poled, temperature sensitive pyroelectric material as the filtering medium. Moreover they do not provide a filtering medium having a prolonged lifetime.